Vacuum holder to conform casting surface

ABSTRACT

A process and apparatus are provided for flattening a metal sheet having a sinuous cross-section while it is supported from below against buckling by withdrawing air from beneath the sheet until atmospheric pressure above the sheet firmly presses it against the support substantially free from undulations and for casting a film of substantially uniform thickness on the exposed surface of the flattened sheet.

1111 3,757,735 [451 Sept. 11, 1973 United States Patent [1 1 Crowe3,482,281 12/1969 425/388 X 3,554,834 l/1971 Bennett et 118/50 UX 1VACUUM HOLDER TO CONFORM CASTING SURFACE [75] Inventor: Gordon ApplingCrowe, Ann Arbor,

Mich.

Primary Examiner-Morris Kaplan [73] Assignee: BASF WyandotteCorporation, A[[grney R()bert M Phipps er a1,

Wyandotte, Mich.

Feb. 2, 1973 [22] Filed:

ABSTRACT [21] Appl. No.: 329,232

Related U.S. Application Data Division of Ser. No. 1 19,522, March 1,1971, Pat. No.

62 l 1 3 728 A process and apparatus are provided for flattening a metalsheet having a sinuous cross-section while it is supported from belowagainst buckling by withdrawing air from beneath the sheet untilatmospheric pressure above the sheet firmly presses it against thesupport substantially free from undulations and for casting a film ofsubstantially uniform thickness on the exposed surface of the flattenedsheet.

[56] References Cited UNITED STATES PATENTS 2,931,064 4/1960Matoba............................... 425/388 5 Claims, 3 DrawingFigures PATENTEDSEPI 1 I975 4 7 llll rl c r 5 FIG! FIG. 2

FIG?) VACUUM HOLDER TO CONFORM CASTING SURFACE This is a division ofapplication Ser. No. ll9,522

filed Mar. 1, 1971, now US. Pat. No. 3,728,777..

This invention relates generally to the casting of films and moreparticularly to an improved apparatus for casting relatively thin filmsof substantially uniform thickness wherein a liquid is spread over asurface and allowed to solidify.

Various methods and apparatus have been suggested for preparing filmsand coatings by spreading a material over a surface while it is liquidand maintaining it in its spread state until it solidifies. Theresulting coating may be left on the surface as a coating or it may beremoved and used as a self-sustaining film. For example, selfsustainingsheets of polyurethane are often formed by casting a liquid reactionmixture over the surface of a relatively thin metal sheet coated withmold release agent and, after chemical reaction and solidification ofthe mixture, removing the resulting film from the metal sheet. In thoseinstances where it is desirable to produce a coated sheet, mold releaseagents are not used and the film is not removed from the sheet.

Of the various processes suggested for making such products the one mostoften used commercially, particularly for preparing polyurethane films,involves pouring a liquid castable reaction mixture over the surface ofthe sheet while it lies on the heated surface of a casting table and,after the resulting film has solidified, removing the metal sheet fromthe casting table. More specifically, in preparing an adherent coating,a relatively thin metal sheet may be sand blasted, coated with a bondingagent and clamped or otherwise securely fastened to the surface of aheated casting table. Metal bars are placed on the sheet to form pictureframe" type mold walls enclosing the area of the sheet to be coated. Thecorrect volume of liquid urethane reaction mixture or other suitablesolidifiable mixture is poured over the surface of the enclosed areawhile it is still sufficiently fluid to seek a common level. Aftersolidification the metal sheet is removed from the table and the film isremoved therefrom.

Such a process is relatively simple and has many advantages but has thedisadvantage of often not producing a film of uniform thicknessparticularly when the metal sheet and the film cast thereon arerelatively thin. Even the highest quality stretcher-levelled thin sheetmetal is not truly flat. It usually hasmany bends, bulges and rippleswhich because of its inherent springiness will remain unless held firmlyagainst a flat surface. Frequently, such a sheet will have waves orundulations of a depth equal to or greater than the thickness of thefilm to be cast on the surface of the sheet. Consequently, the liquidlayer poured over the surface will flow into low spots and producepuddles here and there and leave thin layersof liquid or no liquid atall at the high spots. Attempts have been made to hold the sheet flatwith the bars used as a mold frame but the sheet still has a sinuouscross-section in the area enclosed by the bars. Weights have also beenplaced over the entire sheet with only small points spaced here andthere touching the metal sheet within the area enclosed by the bars, butthe coating has a void for each point which must later be filled bypatching. The product has a patched appearance.

It is therefore an object of this invention to provide an apparatus forcasting films which is devoid of the disadvantages of the prior artprocesses. Another object of the invention is to provide an improvedapparatus for casting films prepared by shaping a liquid and effectingsolidification thereof. Still another object of a the invention is toprovide an apparatus for maintaining a metal sheet having a sinuouscross-section flat while a liquid is cast and solidified thereon. Afurther object of the invention is to provide an apparatus of simple andinexpensive construction adapted to flatten a relatively thin flexiblemetal sheet having a sinuous crosssection. A more specific object of theinvention is to provide a simple but effective apparatus for castingrelatively thin films of synthetic resins or the like of substantiallyuniform thickness over a metal sheet having a surface which under normalconditions is not perfectly flat.

Other objects will become apparent from the following description withreference to the accompanying drawing wherein FIG. 1 is a plan view,partially fragmentary, of one embodiment of the invention;

FIG. 2 is a cross-section taken along the line Il-ll of FIG. 1; and

FIG. 3 is an exaggerated illustration of the edge of a metal sheethaving a sinuous cross-section adapted to be flattened in accordancewith this invention.

The objects of the invention are accomplished, generally speaking, byproviding an apparatus for supporting a sheet having a sinuouscross-section and evacuating the region below the sheet until the sheetbecause of the difference in pressure above and below the sheet ispressed against the support in a flattened condition. In other words,the sheet is supported to prevent buckling downwardly as air iswithdrawn and is consequently straightened or leveled by atmosphericpressure which forces it firmly against the various spaced points of thesupport which are in contact with the underside of the sheet.

In a preferred embodiment of the invention, a thin metal sheet is sandblasted or otherwise cleaned by conventional methods prior to coating. Agasket is adhesively secured around-the periphery of the underside ofthe sheet leaving an open region therebetween. A piece of expanded metalwhich has not been flattened after expansion and has been cut "todimensions corresponding substantially to the dimensions of the .openregion is placed on the surface of a level casting table. The sheet isthen placed over the casting table with the gasket about the expandedmetal and resting on the casting table. A hole is drilled in one comerof the sheet to provide for passage of one end of a tube into the openregion under the expanded sheet. The other end of the tube is attachedto the suction side of a blower, to a vacuum pump or a combinationthereof. Air is withdrawn from the region under the sheet until the airpressure above the sheet is sufficiently greater than that within theregion to firmly press the sheet against the upper most points of theexpanded metal and to flatten the surface of the sheet by removalof anyundulations therein. A coating is then cast over the surface of theflattened sheet by pouring a liquid thereover and continuing to maintainthe vacuum until the liquid has solidified.

Referring now to the drawing, a substantially air impervious rubbergasket 5 is adhesively bound around the periphery of the underside of aflexible relatively thin steel sheet 2. One-half inch wide by 3/16 inchthick foam rubber weather stripping of the type available commerciallyfor weather stripping doors and windows ofa residence may be used toform gasket 5. Most stripping has one surface coated with a pressuresensitive adhesive which may be used to bind the stripping to the sheet.A sheet of unflattened diamond patterned expanded metal 4 having amaximum thickness of about one-eighth inch and a length and width aboutequal to those of the region enclosed by gasket is placed on the surfaceof a heated substantially level casting table 1. The assembly of sheet 2and gasket 5 are positioned over table 1 with gasket 5 surrounding theexpanded metal sheet 4. Steel rods 3 are positioned on the exposedsurface of sheet 2 in the desired configuration and dimensions. Theweight of sheet 2 and rods 3 compress gasket 5 until it is about equalin thickness to the thickness of sheet 4. One end of a suitable tubingis passed through opening 7 into the region of expanded metal sheet 4and is connected at its other end to a means for removing air from theregion, such as an aspirator, vacuum pump, suction fan or the like. Airis withdrawn from the said region until the pressure above sheet 2 issufficiently greater than that below sheet 2 to firmly press sheet 2against sheet 4, remove any undulations therein resulting from itssinuous cross-section and thereby flatten the expanded surface of sheet2. While maintaining less than atmospheric pressure below sheet 2, aliquid mixture which will solidify into a substantially non-porouspolyurethane coating 6 is poured in the required volume over the exposedsurface of sheet 2 within the area defined by rods 3. Aftersolidification of the polyurethane, air is permitted to flow into thespace defined by gasket 5, the rods 3 are removed and sheet 2 is removedfrom the casting table 1. if sheet 2 is used as a casting surface for afilm which is to be removed therefrom and used as a selfsustaining film,sheet 2 and the inner walls of rods 3 are coated with a suitable moldrelease before the reaction mixture is poured over sheet 2 to facilitateseparation of the cast film from sheet 2. Any suitable mold releaseknown in the art may be used such as a wax, a silicone or one of themold release agents disclosed in the Saunders and Frisch book,identified more specifically hereinafter, if the film is to be removedfrom the metal sheet. Any known suitable bonding agent may be used inpreparing an adherent coating on a metal sheet.

The process and apparatus provided by the invention may be used to castany liquid which will solidify into a product which is a solid at 20 C.,such as, for example, molten paraffin wax, beeswax or other wax, aliquid urethane reaction mixture, any molten resinous material or thelike. Suitable mixtures for casting selfsustaining polyurethane films oradherent coatings on a sheet are disclosed, for example, in the bookentitled Polyurethanes: Chemistry and Technology by Saunders and Frisch,published by Interscience Publishers, Copyright 1962, Library ofCongress Card Number 62-18932. Any of the formulations disclosed in thisbook for making coatings or castings may be used. Any other liquid whichwhen poured over the surface of the sheet is sufficiently fluid to seeka common level on a flat surface and will solidify into a film which issolid at 20 C. may be used.

While best results have been obtained so far with an expanded metalsheet any other support which will not block flow of air from under thesheet 2 and will provide sufficient support to prevent buckling orcollapse of sheet 2 when air is withdrawn from under sheet 2 may be usedin combination with an air impervious wall thereabout. For example, alabyrinth formed by positioning metal bars or strips of a thicknesssubstantially equal to the thickness of gasket 5 when it is supportingsheet 2 may be used provided passageways are available for flow of airfrom under sheet 2. Other porous materials such as a sheet of opencelled rigid polyurethane foam or styrene foam may also be used. Usuallythe thickness of gasket 5 and of the air pervious material cooperatingtherewith to support sheet 2 is relatively thin and often a quarter ofan inch or less. Expanded metal sheets are fabricated by first cutting aplurality of slits in a sheet and then stretching the metal to separatethe walls of the slits and form a polygonal shaped opening similar tothat shown in the drawing. The strips of metal between the openings donot all lie in the same plane so sometimes the product is pressed toflatten it before it is used for a screen, a cat-walk, steps or thelike. However, since some thickness is desired and blockage against airflow is to be avoided, unpressed or unflattened expanded metal is usedin this invention. A suitable fitting or plug may be used to provide anair impervious fitting between the tubing and opening 7. Although it ispreferred to adhesively bind gasket 5 to the metal sheet, in someinstances the adhesive can be omitted.

The invention is primarily advantageous for coating sheet metal, suchas, steel, aluminum, brass and the like, but it may also be practiced tocoat any other flexible substrate which has a sinuous cross-section,such as, sheets of various plastic or resinous materials.

in the preferred embodiment of theinvention, described above, air may befirst withdrawn from under sheet 2 with the suction side of a fan typeblower such as a portable vacuum cleaner or similar apparatus capable ofwithdrawing a relatively large volume of air rapidly. This facilitatesseating the gasket against the surface of the casting table and sealingthe region enclosed by the gasket. After the gasket is seated againstthe table, a vacuum pump capable of removing substantially all of theair from under sheet 2 may be cut in to continue withdrawing air untilthe sheet is firmly pressed against the expanded metal sheet and anyundulations have been removed. One arrangement of apparatus suitable forthis embodiment employs a twoway valve in the tubing extending from hole7 with a vacuum cleaner and a vacuum pump attached through suitabletubing to the valve.

The invention is applicable to the coating of relatively large sheets aswell as smaller ones. Sheet steel 4 X 8 feet and about one-sixteenthinch thick and larger may be coated by the process with coatings as thinas one-sixteenth inch or less. The formulations and processing steps inChapter X, Section IV, of the Saunders and Frisch book, Part Il, may beused in combination with the flattening step of this invention.

Although the invention has been described in detail for the purpose ofillustration, it is to be understood that such detail is solely for thatpurpose and that variations can be made by those skilled in the artwithout departing from the spirit and scope of the invention except asit may be limited by the claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination, an apparatus comprisinga support having asubstantially air impervious surface, a substantially air imperviouswall extending upwardly from the support and about a region having anopen top, air pervious means within said region providing a plurality ofpoints spaced upwardly above the impervious surface a distancesubstantially equal to the height of the wall whereby a substantiallyflat cover over the open top would be supported by the wall and thepoints, a flexible sheet having a sinuous cross-section covering theopen top, and means for withdrawing air from the re gion until theundulated sheet is substantially flattened against the support.

2. The apparatus of claim 1 wherein the wall is a gasket adhered to theunder surface of the flexible sheet and the air pervious means is anunflattened, expanded metal sheet.

3. The apparatus of claim 2 wherein the sinuous sheet is metal.

4. The apparatus of claim 3 wherein the means for withdrawing air is acombination of an exhaust fan adapted to remove large volumes of airrapidly and a vacuum pump adapted to reduce the air pressure to a pointwhere it approaches a complete vacuum.

5. In combination, an apparatus comprising a support having asubstantially air impervious surface, a substantially air imperviouswall extending upwardly from the support and about a region having anopen top, air pervious means within said region providing a plurality ofpoints spaced upwardly above the impervious surface a distancesubstantially equal to the height of the wall whereby a substantiallyflat cover over the open top would be supported by the wall and thepoints, a flexible sheet having a sinuous cross-section covering theopen top, means for withdrawing air from the region until the undulatedsheet is substantially flattened against the support, means for heatingthe sheet and a means for casting a film over the surface of the sheet.a: :k

1. In combination, an apparatus comprising a support having asubstantially air impervious surface, a substantially air imperviouswall extending upwardly from the support and about a region having anopen top, air pervious means within said region providing a plurality ofpoints spaced upwardly above the impervious surface a distancesubstantially equal to the height of the wall whereby a substantiallyflat cover over the open top would be supported by the wall and thepoints, a flexible sheet having a sinuous cross-section covering theopen top, and means for withdrawing air from the region until theundulated sheet is substantially flattened against the support.
 2. Theapparatus of claim 1 wherein the wall is a gasket adhered to the undersurface of the flexible sheet and the air pervious means is anunflattened, expanded metal sheet.
 3. The apparatus of claim 2 whereinthe sinuous sheet is metal.
 4. The apparatus of claim 3 wherein themeans fOr withdrawing air is a combination of an exhaust fan adapted toremove large volumes of air rapidly and a vacuum pump adapted to reducethe air pressure to a point where it approaches a complete vacuum.
 5. Incombination, an apparatus comprising a support having a substantiallyair impervious surface, a substantially air impervious wall extendingupwardly from the support and about a region having an open top, airpervious means within said region providing a plurality of points spacedupwardly above the impervious surface a distance substantially equal tothe height of the wall whereby a substantially flat cover over the opentop would be supported by the wall and the points, a flexible sheethaving a sinuous cross-section covering the open top, means forwithdrawing air from the region until the undulated sheet issubstantially flattened against the support, means for heating the sheetand a means for casting a film over the surface of the sheet.